Index: post.php
--- post.php Base (BASE)
+++ post.php Locally Modified (Based On LOCAL)
@@ -757,7 +757,7 @@
 
 		$metakey = wp_slash( $metakey );
 
-		return add_post_meta( $post_ID, $metakey, $metavalue );
+		return update_post_meta( $post_ID, $metakey, $metavalue );
 	}
 
 	return false;